Optical fiber production method

ABSTRACT

An optical fiber production method includes: drawing an optical fiber from an optical fiber preform in a drawing furnace; and cooling the optical fiber. The optical fiber is passed through a plurality of annealing furnaces while the optical fiber is cooled. Equation (1) is held in a given period during the cooling, where a time constant of relaxation of a structure of glass forming a core included in the optical fiber is defined as τ(T), a temperature of the optical fiber at a point in time during the cooling is defined as T, a fictive temperature of glass forming the core at the point in time is defined as T f   0 , and a fictive temperature of glass forming the core after a lapse of time Δt from the point in time is defined as T f . 
       20° C.&lt; T   f   −T =( T   f   0   −T )exp(−Δ t /τ( T ))&lt;100° C.  (1)

TECHNICAL FIELD

The present invention relates to an optical fiber production method.

BACKGROUND

In optical fiber communication systems, in order to increase the reachand the rate of optical transmission, the optical signal-to-noise ratiohas to be increased. Thus, a decrease in transmission losses in opticalfibers is demanded. Nowadays, since an optical fiber production methodis highly sophisticated, transmission losses caused by impuritiescontained in optical fibers are closed to the lower limits. A remainingmain cause of transmission losses is scattering losses in associationwith fluctuations in the structure or composition of glass formingoptical fibers. This is inevitable, because optical fibers are formed ofglass.

As a method of decreasing fluctuations in the structure of glass, amethod is known to cool molten glass slowly. As a method of slowlycooling molten glass, an attempt is made to slowly cool an optical fiberdrawn from a drawing furnace immediately. Specifically, it isinvestigated to decrease the cooling rate of the optical fiber that anoptical fiber drawn from a drawing furnace is heated in an annealingfurnace, or an optical fiber drawn from a drawing furnace is surroundedby a heat insulator immediately.

Patent Literature 1 below discloses a method of setting the temperatureof a heating furnace (an annealing furnace) is ±100° C. or less of thetarget temperature found by a recurrence formula in 70% or more of aregion from a position at which the outer diameter of a silica basedoptical fiber having a core and a cladding becomes smaller than 500% ofthe final outer diameter to a position at which the temperature of theoptical fiber is 1,400° C. Since the temperature history of the opticalfiber is controlled in this manner, the fictive temperature of glassforming the optical fiber is decreased to reduce transmission losses.

-   [Patent Literature 1] JP2014-62021A

SUMMARY

However, the technique disclosed in Patent Literature 1 above isrequired to repeat complex calculations in order to adjust thetemperature of the optical fiber to an ideal temperature change found bythe recurrence formula. The technique disclosed in Patent Literature 1permits the temperature of the optical fiber to have a temperature shiftof as large as ±50° C. to 100° C. with respect to the target temperaturefound by the recurrence formula. When the temperature shift of theoptical fiber is permitted in such a large deviation, it is difficult tosay that the temperature history is sufficiently optimized. For example,supposing that the temperature of the optical fiber slowly cooled ischanged in a range of ±100° C. and the fictive temperature of glassforming the optical fiber is also changed in a similar range,transmission losses of the obtained optical fiber caused by lightscattering are increased or decreased as large as by 0.007 dB/km. Insuch the disclosed production methods in which the temperature historyof the optical fiber is not sufficiently optimized, capital investmentis excessively spent for elongating the annealing furnace more thannecessary, or productivity is degraded by decreasing the drawing ratemore than necessary.

The present inventors found that the transmission losses in the opticalfiber are reduced easily by appropriately setting the temperature of theannealing furnace and appropriately controlling the temperaturedifference between the fictive temperature of glass forming the opticalfiber and the temperature of the optical fiber, because of promoting therelaxation of the structure of glass forming the optical fiber.

One or more embodiments of the present invention provide an opticalfiber production method that easily reduces transmission losses in theoptical fiber.

An optical fiber production method according to one or more embodimentsof the present invention includes: a drawing process of drawing anoptical fiber from an optical fiber preform in a drawing furnace; and aslow cooling process of slowly cooling the optical fiber drawn in thedrawing process. In the slow cooling process, the optical fiber ispassed through a plurality of annealing furnaces. Equation (1) below isheld in a given period in the slow cooling process, where a timeconstant of relaxation of a structure of glass forming a core includedin the optical fiber is defined as τ(T), a temperature of the opticalfiber at a point in time in the slow cooling process is defined as T, afictive temperature of glass forming the core at the point in time isdefined as T_(f) ⁰, and a fictive temperature of glass forming the coreafter a lapse of time Δt from the point in time is defined as T_(f).

20° C.<T _(f) −T=(T _(f) ⁰ −T)exp(−Δt/τ(T))<100° C.  (1)

The present inventors found that the optical fiber is slowly cooled withthe temperature difference between the temperature of the optical fiberand the fictive temperature of glass forming the core included in theoptical fiber being controlled in the predetermined range and hence therelaxation of the structure of glass forming the core is promoted. Withthe promotion of the relaxation of the structure of glass forming thecore, scattering losses caused by fluctuations in the structure of glassforming the core in the transmission of light through the core arereduced, and hence transmission losses in the optical fiber are reduced.As described above, the plurality of annealing furnaces is used in theslow cooling process, the preset temperatures of the annealing furnacesare appropriately controlled, and hence the temperature differencebetween the temperature of the optical fiber and the fictive temperatureof glass forming the core included in the optical fiber is easilycontrolled in the predetermined range. As a result, the relaxation ofthe structure of glass forming the core is promoted, and transmissionlosses in the optical fiber are easily reduced.

In the optical fiber production method according to one or moreembodiments of the present invention, Equation (2) below is held in agiven period in the slow cooling process.

40° C.<T _(f) −T=(T _(f) ⁰ −T)exp(−Δt/τ(T))<60° C.  (2)

In this manner, in the slow cooling process, the temperature difference(T_(f)−T) between the temperature T of the optical fiber and the fictivetemperature T_(f) of glass forming the core included in the opticalfiber is controlled in a more suitable range, and hence the relaxationof the structure of glass forming the core included in the optical fiberis more easily promoted, and transmission losses in the optical fiberare more easily reduced.

In the optical fiber production method according to one or moreembodiments of the present invention, a relationship of Equation (3)below is held, where a preset temperature of an nth annealing furnace ofthe plurality of annealing furnaces from an upstream side is defined asT_(sn) and a fictive temperature of glass forming a core included in theoptical fiber at an outlet port of the nth annealing furnace of theplurality of annealing furnaces from the upstream side is T_(en).

20° C.<T _(en) −T _(sn)<100° C.  (3)

As described above, the plurality of annealing furnaces is used in theslow cooling process, the preset temperatures of the annealing furnacesare controlled in a predetermined range with respect to the fictivetemperature of glass forming the core at the outlet ports of theannealing furnaces, and hence the temperature difference between thetemperature of the optical fiber and the fictive temperature of glassforming the core included in the optical fiber is easily controlled in apredetermined range. As a result, the relaxation of the structure ofglass forming the core is promoted, and transmission losses in theoptical fiber are easily reduced.

In the optical fiber production method according to one or moreembodiments of the present invention, Equation (4) below is held.

40° C.<T _(en) −T _(sn)<60° C.  (4)

In this manner, the preset temperatures of the plurality of theannealing furnaces are individually controlled in a more suitable range,and hence the effect of promoting the relaxation of the structure ofglass forming the core included in the optical fiber is easilyincreased, and transmission losses in the optical fiber are more easilyreduced.

In the optical fiber production method according to one or moreembodiments of the present invention, a temperature difference between apreset temperature and a fictive temperature of glass forming the coreat an outlet port is smaller in the annealing furnace provided on adownstream side than in the annealing furnace provided on an upstreamside.

The present inventors found that when the temperature of glass becomeslow, a small temperature difference between the fictive temperature ofglass and the temperature of glass easily promotes the relaxation of thestructure of glass. Thus, the temperature of the annealing furnace isset so that the temperature difference between the preset temperatureand the fictive temperature of glass forming the core at the outlet portis smaller in the annealing furnace provided on the downstream side thanin the annealing furnace provided on the upstream side. Thus, therelaxation of the structure of glass forming the core can be efficientlypromoted. As a result, transmission losses in the optical fiber are moreeasily reduced.

In one or more embodiments, the optical fiber is in any one of theplurality of annealing furnaces during at least certain period for whicha temperature of the optical fiber is in a range of 1,300° C. or moreand 1,500° C. or less.

The optical fiber is slowly cooled when the temperature of the opticalfiber is in this range, and hence the fictive temperature of glassforming the core included in the optical fiber is easily decreased for ashorter time, and transmission losses in the optical fiber are easilyreduced.

As described above, according to one or more embodiments of the presentinvention, an optical fiber production method that easily reducestransmission losses in the optical fiber is provided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flowchart of the processes of an optical fiber productionmethod according to one or more embodiments of the present invention.

FIG. 2 is a schematic diagram of the configuration of devices for use inan optical fiber production method according to one or more embodimentsof the present invention.

FIG. 3 is a graph of the relationship of the temperature of glass andthe fictive temperature of the glass with slow cooling time according toone or more embodiments of the present invention.

FIG. 4 is a graph showing the relationship of the temperature difference(T_(f) ⁰−T) between the fictive temperature of glass and the temperatureof glass with the decrease rate ((T_(f)−T_(f) ⁰)/Δt) of the fictivetemperature of glass per unit time according to one or more embodimentsof the present invention.

FIG. 5 is a graph of a temporal change in the temperature differencebetween the fictive temperature of glass and the temperature of glassaccording to one or more embodiments of the present invention.

FIG. 6 is a graph of the upper limit and the lower limit of thevariation over time in the optimized temperature difference (T_(f)−T)depicted by a solid line in FIG. 5 and the temperature difference(T_(f)−T) in which a transmission loss caused by scattering is notincreased by 0.001 dB/km or more according to one or more embodiments ofthe present invention.

FIG. 7 is a graph of the preset temperatures of annealing furnaces, theoptimized fictive temperature of glass at the outlet ports of theannealing furnaces, and the fictive temperature of glass through thefictive temperature history at the outlet ports of the annealingfurnaces according to one or more embodiments of the present invention.

DETAILED DESCRIPTION

In the following, an optical fiber production method according to one ormore embodiments of the present invention will be described in detailwith reference to the drawings.

FIG. 1 is a flowchart of the processes of an optical fiber productionmethod according to one or more embodiments of the present invention. Asillustrated in FIG. 1, the optical fiber production method according toone or more embodiments includes a drawing process P1, a precoolingprocess P2, a slow cooling process P3, and a rapid cooling process P4.In the following, these processes will be described. Note that, FIG. 2is a schematic diagram of the configuration of devices for use in theoptical fiber production method according to one or more embodiments.

<Drawing Process P1>

The drawing process P1 is a process in which one end of an optical fiberpreform 1P is drawn in a drawing furnace 110. First, the optical fiberpreform 1P is prepared. The optical fiber preform 1P is formed of glasshaving refractive index profiles the same as the refractive indexprofiles of a core and a cladding forming an optical fiber 1. Theoptical fiber 1 includes one or a plurality of cores and a claddingsurrounding the outer circumferential surface of the core with no gap.The core and the cladding are formed of silica glass. The refractiveindex of the core is higher than the refractive index of the cladding.For example, in the case in which the core is formed of silica glassdoped with a dopant, such as germanium, which increases the refractiveindex, the cladding is formed of pure silica glass. For example, in thecase in which the core is formed of pure silica glass, the cladding isformed of silica glass doped with a dopant, such as fluorine, whichdecreases the refractive index.

Subsequently, the optical fiber preform 1P is suspended so that thelongitudinal direction is perpendicular. The optical fiber preform 1P isdisposed in the drawing furnace 110, a heating unit 111 is caused togenerate heat, and then the lower end portion of the optical fiberpreform 1P is heated. At this time, the lower end portion of the opticalfiber preform 1P is heated at a temperature of 2,000° C., for example,to be molten. From the heated lower end portion of the optical fiberpreform 1P, molten glass is drawn out of the drawing furnace 110 at apredetermined drawing rate.

<Precooling Process P2>

The precooling process P2 is a process in which the optical fiber drawnout of the drawing furnace 110 in the drawing process P1 is cooled to apredetermined temperature suitable for delivering the optical fiber intoan annealing furnace 121, described later. A predetermined temperatureof the optical fiber suitable for delivering the optical fiber into theannealing furnace 121 will be described later in detail.

In the optical fiber production method according to one or moreembodiments, the precooling process P2 is performed by passing theoptical fiber drawn in the drawing process P1 through the hollow portionof a tubular body 120 provided directly below the drawing furnace 110.The tubular body 120 is provided directly below the drawing furnace 110,causing the atmosphere in the inside of the hollow portion of thetubular body 120 to be almost the same as the atmosphere in the insideof the drawing furnace 110. Thus, a sudden change in the atmosphere andthe temperature around the optical fiber immediately after drawn isreduced.

The temperature of the optical fiber to be delivered into the annealingfurnace 121 is mainly determined by the drawing rate and the atmospherein the drawing furnace 110. The precooling process P2 is provided, whichfurther finely adjusts the cooling rate of the optical fiber for easyadjustment of the incoming temperature of the optical fiber to bedelivered into the annealing furnace 121 to a suitable range. Based onthe temperatures of the optical fiber drawn out of the drawing furnace110 and suitable for delivering the optical fiber into the annealingfurnace 121, the distance from the annealing furnace 121 to the drawingfurnace 110 and the length of the tubular body 120 can be appropriatelyselected. The tubular body 120 is formed of a metal tube, for example.The cooling rate of the optical fiber may be adjusted by air-cooling themetal tube or by providing a heat insulator around the metal tube.

<Slow Cooling Process P3>

The slow cooling process P3 is a process in which the optical fiber,which is drawn out in the drawing process P1, is slowly cooled. In theoptical fiber production method according to one or more embodiments,the temperature of the optical fiber is adjusted through the precoolingprocess P2, and then the optical fiber is slowly cooled in the slowcooling process P3. In the slow cooling process P3, the optical fiber ispassed through a plurality of annealing furnaces 121 a, 121 b, 121 c,and 121 d. In the description of the optical fiber production methodaccording to one or more embodiments, in the case in which all theannealing furnaces are collectively referred or in the case in which itis unnecessary to distinguish between the annealing furnaces, they aresometimes simply referred to as “the annealing furnace 121”. Note that,in FIG. 2, four annealing furnaces 121 a, 121 b, 121 c, and 121 d areshown. However, in one or more embodiments of the present invention, thenumber of the annealing furnaces is not limited specifically as long asthe number is more than one. The presence of more than one annealingfurnace means that there is a plurality of heat generating units whosetemperatures can be set differently. For example, it can be said that aplurality of annealing furnaces is present when a plurality of heatgenerating units whose temperatures can be set differently is providedeven though the heat generating units are housed in one enclosure.

The temperature in the inside of the annealing furnace 121 is apredetermined temperature different from the temperature of the opticalfiber to be delivered into the annealing furnace 121. The cooling rateof the optical fiber is decreased by the temperature around the opticalfiber delivered into the annealing furnace 121. The cooling rate of theoptical fiber is decreased in the annealing furnace 121. Thus, thestructure of glass forming the core included in the optical fiber isrelaxed, and the optical fiber 1 with decreased scattering losses isobtained, as described below.

In the disclosed optical fiber production methods having the slowcooling process, the temperature of the optical fiber is notsufficiently optimized when the optical fiber is delivered into theannealing furnace. Specifically, the optical fiber is sometimesdelivered into the annealing furnace with the temperature of the opticalfiber being too high or too low. When the temperature of the opticalfiber to be delivered into the annealing furnace is too high, the rateto relax the structure of glass forming the optical fiber is too fast,hardly expecting the effect of slowly cooling the optical fiber. On theother hand, when the temperature of the optical fiber to be deliveredinto the annealing furnace is too low, the rate to relax the structureof glass forming the optical fiber is decreased, sometimes causing anecessity to heat up again the optical fiber in the annealing furnace,for example. As described above, in the disclosed slow coolingprocesses, it is difficult to say that the relaxation of the structureof glass forming the optical fiber is efficiently performed. Thus, theannealing furnace is elongated more than necessary, which might demandan excessive capital investment, or the drawing rate is decreased morethan necessary, which might degrade productivity.

According to the optical fiber production method of one or moreembodiments, in the slow cooling process P3, the temperature of theannealing furnace 121 is appropriately set, the temperature differencebetween the fictive temperature of glass forming the core included inthe optical fiber and the temperature of the optical fiber isappropriately controlled, and hence the relaxation of the structure ofglass forming the core is promoted. As a result, the optical fiber 1having decreased transmission losses can be obtained with no requirementof excessive capital investment and with excellent productivity.According to the optical fiber production method of one or moreembodiments, complex calculation is unnecessary unlike the techniquedisclosed in Patent Literature 1 described above.

In silica glass classified as so-called strong glass, the time constantτ(T) of the structural relaxation, which is thought to correspond to theviscosity flow of glass, follows the Arrhenius equation. Thus, the timeconstant τ(T) is expressed as Equation (5) as a function of thetemperature T of glass using a constant A and an activation energyE_(act) determined by the composition of glass. Note that, k_(B) isBoltzmann constant.

1/τ(T)=A·exp(−E _(act) /k _(B) T)  (5)

(Here, T is absolute temperature of glass.)

Equation (5) above shows that the structure of glass is relaxed fasteras the temperature of glass is higher and reached faster in theequilibrium state at the given temperature. That is, the fictivetemperature of glass more quickly comes close to the temperature ofglass faster as the temperature of glass is higher.

FIG. 3 shows the relationship of the temperature of glass and thefictive temperature of the glass with time in slowly cooling glass. Inthe graph of FIG. 3, the horizontal axis expresses time, and thevertical axis expresses temperature. In FIG. 3, a solid line expressesthe transition of the temperature of glass under certain slow coolingconditions, and a broken line expresses the transition of the fictivetemperature of glass at that time. A dotted line expresses thetransition of the temperature of glass in the case in which the coolingrate is decreased more slowly than under the slow cooling conditionsexpressed by the solid line, and an alternate long and short dash lineexpresses the transition of the fictive temperature of glass at thattime.

As expressed by the solid line and the broken line in FIG. 3, when thetemperature of glass is decreased over a lapse of time in the hightemperature area, the fictive temperature of glass is also similarlydecreased. As described above, in the state in which the temperature ofglass is sufficiently high, the rate of the relaxation of the structureof glass forming the optical fiber is very fast. However, as thetemperature of glass is decreased, the rate of the relaxation of thestructure of glass is decreased, and the fictive temperature of glassfails to follow a decrease in the temperature of glass after a while.The temperature difference between the temperature of glass and thefictive temperature of glass is then increased. Here, when the coolingrate of glass is slowed, the optical fiber is held in a relativelyhigher temperature state for a longer time, compared with the case inwhich the cooling rate is fast. Thus, as expressed by the dotted lineand the alternate long and short dash line in FIG. 3, the temperaturedifference between the temperature of glass and the fictive temperatureof glass becomes smaller, and the fictive temperature of glass is lowerthan the example described above. That is, when the cooling rate ofglass is slowed, the relaxation of the structure of glass is easilypromoted.

As described above, when the temperature of glass is high, the structureof glass is relaxed fast. However, the fictive temperature of glass doesnot reach to the below of the temperature of glass. Thus, when thetemperature of glass is high, the fictive temperature of the glass alsoremains high. That is, when the temperature of glass is too high, theeffects obtained by slow cooling are poor. From this viewpoint, thetemperature of the optical fiber in the annealing furnace 121 is 1,600°C. or less, or 1,500° C. or less. On the other hand, in the case inwhich the temperature of glass is low, the fictive temperature can bedecreased to a lower temperature, but the decrease rate of the fictivetemperature is slowed. That is, when the temperature of glass is toolow, it will take longer time for slow cooling in order to sufficientlydecrease the fictive temperature. From this viewpoint, the temperatureof the optical fiber in the annealing furnace 121 is 1,300° C. or more,or 1,400° C. or more. Therefore, the optical fiber stays in theannealing furnace 121 at least one period during which the temperatureof the optical fiber is in a range of temperatures of 1,300° C. to1,500° C., both inclusive. As described above, in the slow coolingprocess P3, the optical fiber is slowly cooled when the temperature ofthe optical fiber is in a predetermined range. Thus, the fictivetemperature of glass forming the core included in the optical fiber iseasily decreased for a shorter time, and transmission losses in theoptical fiber are easily reduced.

Next, the following is the description in which the relaxation of thestructure of glass forming the core is efficiently promoted to reducetransmission losses in the optical fiber by what manner of slowlycooling the optical fiber by means of the relationship between thetemperature of glass and the fictive temperature of glass.

Under the conditions in which the time constant of the relaxation of thestructure of glass forming the core included in the optical fiber isdefined as τ(T), the temperature of the optical fiber at a certain pointin time in the slow cooling process P3 is defined as T, and the fictivetemperature of glass forming the core at that certain point in time isdefined as T_(f) ⁰, the fictive temperature T_(f) of glass forming thecore after a lapse of time Δt from the certain point in time isexpressed as Equation (6) below based on Equation (5) above. Note that,Δt is a short period of time, and the temperature of the optical fiber Tfor this period is supposed to be constant.

T _(f) −T=(T _(f) ⁰ −T)exp(−Δt/τ(T))  (6)

Equation (6) above shows that the temperature difference (T_(f)−T)between the fictive temperature T_(f) of glass forming the core and thetemperature T of the optical fiber depends on the temperature difference(T_(f) ⁰−T) between the fictive temperature T_(f) ⁰ of glass forming thecore and the temperature T of the optical fiber at a certain point intime as well as the fictive temperature T_(f) of glass forming the coredepends on the time constant τ(T) of the relaxation of the structure.The time constant τ(T) of the relaxation of the structure is defined astime until the temperature difference (T_(f)−T) between the fictivetemperature T_(f) of glass and the temperature T of glass reaches 1/ewhen the temperature of glass whose fictive temperature is T_(f) ⁰ is T.A change in the fictive temperature T_(f) per unit time is greater asthe temperature difference (T_(f) ⁰−T) is greater to some extent.

FIG. 4 schematically shows the relationship between the temperaturedifference (T_(f) ⁰−T) where the temperature of the optical fiberincluding the core formed of glass whose fictive temperature is T_(f) ⁰is T and a change ((T_(f)-) T_(f) ⁰/Δt) in the fictive temperature T_(f)per unit time. As shown in FIG. 4, under the conditions in which thefictive temperature T_(f) ⁰ of glass forming the core coincides with thetemperature T of the optical fiber (T_(f) ⁰=T), the relaxation of thestructure of glass forming the core does not occur, and a change in thefictive temperature per unit time is zero ((T_(f)−T_(f) ⁰)/Δt=0). Theconditions are thought in which the temperature T of the optical fiberis decreased from this point and the temperature difference (T_(f) ⁰−T)between the fictive temperature T_(f) ⁰ of glass forming the core andthe temperature T of the optical fiber is increased. Under theconditions, although the time constant τ(T) of the relaxation of thestructure of glass forming the core is increased, the change rate of thefictive temperature T_(f) per unit time ((T_(f)−T_(f) ⁰)/Δt) isnegatively increased. However, the conditions are thought in which thetemperature T of the optical fiber is further decreased and thetemperature difference (T_(f) ⁰−T) between the fictive temperature T_(f)⁰ of glass forming the core and the temperature T of the optical fiberis further increased. Under the conditions, the time constant τ(T) ofthe relaxation of the structure of glass forming the core is nowincreased, and the absolute value of a change in the fictive temperatureT_(f) per unit time ((T_(f)−T_(f) ⁰)/Δt) is decreased. That is, FIG. 4shows that as a peak expressed in the graph, a change in the fictivetemperature per unit time ((T_(f)−T_(f) ⁰)/Δt) takes a minimum valuewhen the temperature difference (T_(f) ⁰−T) between the fictivetemperature T_(f) ⁰ of glass forming the core and the temperature T ofthe optical fiber is a certain value.

Here, solving Equation (6) above shows that the relationship of Equation(7) below is held between the temperature T of glass and the fictivetemperature T_(f) when the decrease rate of the fictive temperatureT_(f) of glass is the maximum.

T ²+(E _(act) /k _(B))×T−(E _(act) /k _(B))×T _(f) =O  (7)

When Equation (7) above is further solved on T as Equation (8) below,the temperature T of glass can be found, at which the fictivetemperature T_(f) of glass can be most efficiently decreased. In thefollowing, the temperature of glass, at which the fictive temperatureT_(f) of glass can be most efficiently decreased, is sometimes referredto as “the optimized temperature of glass”, and the fictive temperaturethat has been most efficiently decreased is sometimes referred to as“the optimized fictive temperature”.

$\begin{matrix}{T = \frac{{- \frac{E_{act}}{k_{B}}} + {\sqrt{\left( \frac{E_{act}}{k_{B}} \right)^{2} + {4\frac{E_{act}}{k_{B}}}}T_{f}}}{2}} & (8)\end{matrix}$

As described so far, when the temperature difference (T_(f) ⁰−T) betweenthe fictive temperature T_(f) ⁰ of glass and the temperature T of glassat a certain point in time is a predetermined value, a change in thefictive temperature T_(f) of glass per unit time is maximized. That is,when the fictive temperature T_(f) after a lapse of a certain time Δt ofglass having the fictive temperature T_(f) ⁰ is thought, the temperatureT of glass is present at which fictive temperature T_(f) can be minimumvalue.

FIG. 5 shows, on a standard single-mode optical fiber having a coredoped with G_(e)O₂, a variation over time of the temperature difference(T_(f)−T) between the value where the fictive temperature T_(f) of glassforming the core, which is found from Equation (6) above, takes thelowest value and the temperature T of the optical fiber at that value.In the graph shown in FIG. 5, the vertical axis expresses thetemperature difference (T_(f)−T) between the value where the fictivetemperature T_(f) of glass forming the core takes the lowest value andthe temperature T of the optical fiber at that value, and the horizontalaxis expresses the slow cooling time of the optical fiber. The graphexpressed by a solid line is the result using the constant A and theactivation energy E_(act) described in Non-Patent Literature 1 (K.Saito, et al., Journal of the American Ceramic Society, Vol. 89, pp.65-69 (2006)), and the graph expressed by a broken line is the resultusing the constant A and the activation energy E_(act) described inNon-Patent Literature 2 (K. Saito, et al., Applied Physics Letters, Vol.83, pp. 5175-5177 (2003)) and Δt is 0.0005 second.

Here, it is supposed that the slow cooling process P3 is performedimmediately after the optical fiber preform 1P is heated and molten inthe drawing process P1. Supposing that a temperature T⁰ of the opticalfiber is 1,800° C. at the beginning of slow cooling, at which slowcooling time is zero second, time required for relaxing the structure ofglass forming the core at this temperature is as very short as less than0.001 second. Thus, it can be thought that the fictive temperature T_(f)⁰ of glass forming the core at the beginning of slow cooling is also1,800° C. That is, the initial value is assumed as T_(f) ⁰−T⁰=0° C.

Regarding the variation over time of the temperature difference(T_(f)−T) between the fictive temperature of glass forming the core andthe temperature of the optical fiber derived from the assumption, it isshown that in the time domain up to about 0.01 second, the temperaturedifference (T_(f)−T) has to be gradually increased, whereas in the timedomain from about 0.01 second and later, the temperature difference(T_(f)−T) has to be gradually decreased. It is shown that in all thetime domains, the temperature difference (T_(f)−T) has to be less thanabout 60° C., the temperature T of the optical fiber is controlled sothat in almost all the time domains, the temperature difference(T_(f)−T) is kept higher than about 40° C. and less than about 60° C.,and hence the fictive temperature T_(f) of glass forming the core isefficiently decreased. Time at which the temperature difference(T_(f)−T) shown in FIG. 5 is the maximum, is about 0.01 second, althoughthe time is varied more or less depending on the constant A and theactivation energy E_(act) in Equation (5) above, and the temperature T⁰of the optical fiber and the fictive temperature T_(f) ⁰ of glassforming the core at the beginning of slow cooling, at which slow coolingtime is zero second.

The assumption shows that the precooling process P2 is providedsubsequent to the drawing process P1 so that the temperature difference(T_(f)−T) between the fictive temperature of glass forming the core andthe temperature of the optical fiber is provided to some extent, andthen the slow cooling process P3 is performed, which allows theefficient relaxation of the structure of glass forming the core with theadvantageous use of the length of the annealing furnace 121. Forexample, the precooling process P2 is performed until a point in time atwhich the temperature difference (T_(f)−T) between the fictivetemperature of glass forming the core and the temperature of the opticalfiber is reached at above 40° C. and less than 60° C. at a point in timeafter a lapse of about 0.01 second FIG. 5, and then the slow coolingprocess P3 is started so that the length of the annealing furnace 121can be advantageously used.

The results shown in FIG. 5 reveal the following. It is revealed thateven though slight differences are present in the values of the constantA and the activation energy E_(act) determined based on the compositionof glass, when the temperature difference (T_(f)−T) between the fictivetemperature of glass and the temperature of glass is in a range of above40° C. and less than 60° C. in the slow cooling process P3, the fictivetemperature of glass is efficiently decreased. Thus, in so-calledtypical optical fibers in which the concentration of dopant is low andits principal component is silica glass, the optical fiber is slowlycooled under the conditions in which the temperature difference(T_(f)−T) between the fictive temperature of glass forming the opticalfiber and the temperature of the optical fiber is in a range of above40° C. and less than 60° C., and hence the fictive temperature of glassforming the optical fiber is efficiently decreased. For example, also incores made of silica glass doped with a dopant, such as G_(e)O₂, andcladdings substantially made of pure silica glass, the fictivetemperature is efficiently decreased.

In a given period from the start to the end of the slow cooling processP3, the temperature difference (T_(f)−T) between the temperature T ofthe optical fiber and the fictive temperature T_(f) of glass forming thecore included in the optical fiber is controlled in the predeterminedrange, and hence the relaxation of the structure of glass forming thecore included in the optical fiber is easily promoted and transmissionlosses in the optical fiber are easily reduced. That is, when the timeconstant of the relaxation of the structure of glass forming the core isdefined as τ(T), the temperature of the optical fiber at a certain pointin time in the slow cooling process P3 is defined as T, the fictivetemperature of glass forming the core at that certain point in time isdefined as T_(f) ⁰, and the fictive temperature of glass forming thecore after a lapse of time Δt from the certain point in time is definedas T_(f), Equation (2) below is held.

40° C.<T _(f) −T=(T _(f) ⁰ −T)exp(−Δt/τ(T))<60° C.  (2)

As described above, in the slow cooling process P3, the temperaturedifference (T_(f)−T) between the temperature T of the optical fiber andthe fictive temperature T_(f) of glass forming the core included in theoptical fiber is controlled in a predetermined range, and hence therelaxation of the structure of glass forming the core included in theoptical fiber is more easily promoted. Therefore, transmission losses inthe optical fiber are easily reduced.

At this time, according to the optical fiber production method of one ormore embodiments, in the slow cooling process P3, the plurality of theannealing furnaces 121 is used, the preset temperatures of the annealingfurnaces 121 are appropriately controlled, and hence the temperaturedifference (T_(f)−T) between the temperature of the optical fiber andthe fictive temperature of glass forming the core included in theoptical fiber is easily controlled in a predetermined range. As aresult, the relaxation of the structure of glass forming the core ispromoted, and transmission losses in the optical fiber are reduced.

Note that, the conditions for the temperature difference (T_(f)−T)between the temperature T of the optical fiber and the fictivetemperature T_(f) of glass forming the core included in the opticalfiber in order to most efficiently decrease the fictive temperatureT_(f) of glass forming the core are as described above. However,transmission losses in the optical fiber can also be sufficientlyreduced under the conditions described below.

The fictive temperature T_(f) of glass forming the core included in theoptical fiber can be tied to transmission losses in the optical fiber bya relational expression below. A Rayleigh scattering coefficient R_(r)is proportional to the fictive temperature T_(f) of glass forming thecore, and a transmission loss α_(T) caused by Rayleigh scattering isexpressed by Equation (9) below where the wavelength of light to betransmitted is A (μm).

α_(T) =R _(r)/λ⁴ =BT _(f)/λ⁴  (9)

Here, based on Non-Patent Literature 2 (K. Saito, et al., AppliedPhysics Letters, Vol. 83, pp. 5175-5177 (2003)), B=4.0×10⁻⁴ dB/km/μm⁴/K.Let us consider a transmission loss at the wavelength λ=1.55 μm. Whenthe fictive temperature T_(f) of glass forming the core is increased by14° C., the Rayleigh transmission loss α_(T) caused by Rayleighscattering is increased by about 0.001 dB/km. That is, when errors fromthe fictive temperature T_(f) of glass forming the core, at which thefictive temperature T_(f) is most efficiently decreased, can berestricted to less than 14° C., an increase in the Rayleigh transmissionloss α_(T) caused by Rayleigh scattering can be controlled to less than0.001 dB/km.

As described above, in the case of taking into account of permissiveerrors based on the fictive temperature T_(f) of glass forming the core,at which the fictive temperature T_(f) is most efficiently decreased,the optical fiber only has to be delivered into the annealing furnace121 under the temperature conditions in which the temperature difference(T_(f)−T) between the fictive temperature T_(f) of glass forming thecore and the temperature of the optical fiber is higher than 20° C. andlower than 100° C. as described below.

The temperature difference, at which an increase in the scattering lossexpected from the fictive temperature T_(f) of glass forming the coreafter a lapse of a slow cooling time of 0.5 second at the temperaturedifference (T_(f)−T) expressed by the solid line in FIG. 5 can berestricted to less than 0.001 dB/km, can be predicted from Recurrenceformula (6) above. It is assumed that the temperature T_(f) ⁰ of theoptical fiber at the beginning of slow cooling, at which slow coolingtime is zero second, is 1,800° C. and the temperature difference(T_(f)−T) is almost constant during the slow cooling process P3.Recurrence formula (6) is solved, and then a graph shown in FIG. 6 isobtained. In FIG. 6, the temperature difference (T_(f)−T) expressed bythe solid line in FIG. 5 is again expressed by a solid line. FIG. 6shows the upper limit expressed by a broken line and the lower limitexpressed by an alternate long and short dash line of a variation overtime of the temperature difference (T_(f)−T) at which the transmissionloss caused by scattering is not increased by 0.001 dB/km or more. Here,for the constant A and the activation energy E_(act), the valuesdescribed in Non-Patent Literature 1 (K. Saito, et al., Journal of theAmerican Ceramic Society, Vol. 89, pp. 65-69 (2006)) are used. Theresult shown in FIG. 6 reveals the following. When the temperature ofthe annealing furnace 121 only has to be set so as to control thetemperature history of the optical fiber in which the temperaturedifference (T_(f)−T) is in a range of above about 20° C. and less thanabout 100° C. in the time domain from about 0.01 second and later duringthe slow cooling process P3, an increase in the fictive temperature ofglass forming the core is restricted up to a rise of about 14° C. withrespect to the fictive temperature T_(f) of glass forming the core, atwhich the fictive temperature T_(f) is most efficiently decreased. As aresult, an increase can be restricted to an increase of 0.001 dB/km orless with respect to the values under the optimized conditions underwhich transmission losses are most decreased.

Thus, the temperature difference (T_(f)−T) between the temperature T ofthe optical fiber and the fictive temperature T_(f) of glass forming thecore included in the optical fiber is maintained in a range of above 20°C. and less than 100° C. also in a given period from the start to theend of the slow cooling process P3, and hence the relaxation of thestructure of glass forming the core included in the optical fiber iseasily promoted, and transmission losses in the optical fiber are easilyreduced. That is, Equation (1) below is held.

20° C.<T _(f) −T=(T _(f) ⁰ −T)exp(−Δt/τ(T))<100° C.  (1)

Next, a specific example for easily satisfying the conditions ofEquation (2) or (1) above will be described. In the optical fiberproduction method according to one or more embodiments, four annealingfurnaces 121 a, 121 b, 121 c, and 121 d are used in the slow coolingprocess P3. The plurality of the annealing furnaces 121 is used in thismanner, and hence the temperature difference between the temperature ofthe optical fiber and the fictive temperature of glass forming the coreis easily controlled in a predetermined range. That is, in the slowcooling process P3, when the optical fiber is passed through theplurality of the annealing furnaces 121, the preset temperature of thenth annealing furnace 121 from the upstream side is defined as T_(sn),and the fictive temperature T_(f) of glass forming the core in slowcooling time until which the optical fiber is reached at the outlet portof the nth annealing furnaces 121 from the upstream side is defined asT_(sn), the relationship of Equation (3) below is to be held.

20° C.<T _(en) −T _(sn)<100° C.  (3)

As described above, the optical fiber is slowly cooled with thetemperature difference between the temperature of the optical fiber andthe fictive temperature of glass forming the core included in theoptical fiber being controlled in a predetermined range, and hence therelaxation of the structure of glass forming the core is promoted. Withthe promotion of the relaxation of the structure of glass forming thecore, scattering losses caused by fluctuations in the structure of glassforming the core in the transmission of light through the core arereduced, and hence transmission losses in the optical fiber are reduced.As described above, in the slow cooling process P3, the plurality of theannealing furnaces 121 is used, and the preset temperatures of theannealing furnaces 121 is controlled in a predetermined range withrespect to the fictive temperature of glass forming the core in slowcooling time until which the optical fiber is reached at the outlet portof each of the annealing furnaces 121, and hence the temperaturedifference between the temperature of the optical fiber and the fictivetemperature of glass forming the core included in the optical fiber iseasily controlled in a predetermined range. As a result, the relaxationof the structure of glass forming the core is promoted, and transmissionlosses in the optical fiber are reduced. Referring to FIG. 7, this willbe described more in detail below.

FIG. 7 shows a change in the optimized fictive temperature of glassforming the core calculated from Equation (5) where the temperature andfictive temperature of the optical fiber are 1,800° C. as the initialvalues (a solid line), the preset temperatures of the annealing furnaces121 a, 121 b, 121 c, and 121 d (an alternate long and short dash line),and the expected fictive temperature of glass forming the core in slowcooling time until which the optical fiber is reached at the outletports of the annealing furnaces 121 a, 121 b, 121 c, and 121 d. In theexample shown in FIG. 7, it is assumed that the lengths of the annealingfurnaces 121 are each 0.5 m and the drawing rate is 20 m/second. Asshown by solid triangles in FIG. 7, the optimized fictive temperatureT_(f) of glass forming the core when the optical fiber is delivered outof each of the annealing furnaces 121, i.e. when the slow cooling timeis 0.025 second, 0.050 second, 0.075 second, and 0.100 second, theoptimized fictive temperature T_(f) is calculated as temperatures of1,556° C., 1,515° C., 1,492° C., and 1,476° C., respectively. The presettemperatures of the annealing furnaces 121 a, 121 b, 121 c, and 121 dare then set as expressed by the alternate long and short dash line inFIG. 7. That is, the temperatures of the annealing furnaces 121 are setto a temperature lower by 50° C. than the optimized fictive temperatureT_(f) of glass forming the core at the slow cooling time at which theoptical fiber is reached at the outlet port of each of the annealingfurnaces 121. As a result, since the temperature of the optical fiber isclose to the preset temperatures of the annealing furnaces 121 near theoutlet ports of the annealing furnaces 121, the conditions of Equation(2) or (1) above are easily satisfied near the outlet ports of theannealing furnaces 121. With the sudden change in the temperature ofglass forming the optical fiber delivered into the annealing furnaces121, which is immediately almost comparable with the preset temperaturesof the annealing furnaces 121, the temperature of the glass temporarilydeviates from the conditions of Equation (1). The glass through suchfictive temperature history is expected to have the fictive temperaturesexpressed by solid circles in FIG. 7.

Since the actual temperature of glass is more gently decreased and closeto the preset temperature of the annealing furnace, the actual fictivetemperature is slightly higher than optimized fictive temperaturesexpressed by the solid triangles and slightly lower than the fictivetemperatures expressed by the solid circles. However, this is errors ina tolerable range. In the example shown in FIG. 7, the temperaturedifference between the fictive temperature of glass through a fictivetemperature history after slow cooling for 0.100 second and theoptimized fictive temperature is 12° C., which the different in thescattering loss is only less than 0.001 dB/km.

Based on the viewpoint described above, from the viewpoint ofcontrolling the temperature difference between the temperature of theoptical fiber and the fictive temperature of glass forming the core in amore appropriate range, i.e. from the viewpoint of easily satisfyingEquation (2) above, Equation (4) below is held.

40° C.<T _(en) −T _(sn)<60° C.  (4)

In this manner, the preset temperature of the annealing furnace 121 iscontrolled in a more appropriate range, and hence the effect ofpromoting the relaxation of the structure of glass forming the coreincluded in the optical fiber is easily increased, and transmissionlosses in the optical fiber are easily reduced.

As illustrated in FIG. 5, when the temperature of glass becomes low, asmall temperature difference between the fictive temperature of glassand the temperature of glass easily promotes the relaxation of thestructure of glass. Thus, the temperature difference between the presettemperature and the fictive temperature of glass forming the core at theoutlet port is smaller in the annealing furnace 121 provided on thedownstream side than in the annealing furnace 121 provided on theupstream side. For example, as expressed by the solid line in FIG. 5,the temperature difference between the optimized temperature of glassand the optimized fictive temperature of glass forming the core at slowcooling time of 0.025 second, 0.050 second, 0.075 second, and 0.100second is temperatures of 59° C., 56° C., 55° C., and 54° C.,respectively. The temperature difference is smaller toward thedownstream side. As described above, the temperature of the annealingfurnace is set so that the temperature difference between the presettemperature and the fictive temperature of glass forming the core at theoutlet port is smaller in the annealing furnace provided on thedownstream side than in the annealing furnace provided on the upstreamside. Thus, the relaxation of the structure of glass forming the corecan be efficiently promoted. As a result, transmission losses in theoptical fiber are more easily reduced.

Note that, the relationship between the temperature T of the opticalfiber and the fictive temperature T_(f) of glass forming the core, atwhich the fictive temperature T_(f) is most efficiently decreased,depends solely on the slow cooling time t, and the slow cooling time t,the length L of the annealing furnace, and the drawing rate v can becorrelated with one another based on the relationship of Equation (10)below.

t=L/v  (10)

Therefore, when the targeted fictive temperature T_(f) of glass formingthe core included in the optical fiber to be manufactured is set and thedrawing rate v taking into account of productivity is determined, anecessary length L of the annealing furnace is derived. For example, theslow cooling time t needs about 0.1 second to set the fictivetemperature T_(f) to 1,500° C. Thus, it is revealed that in the case inwhich the drawing rate v is set to 20 m/second, the length L of theannealing furnace needs two meters. For example, the slow cooling time tneeds about 0.4 second in order to set the fictive temperature T_(f) to1,400° C., for example. Thus, it is revealed that in the case in whichthe drawing rate v is set to 10 m/second, the length L of the annealingfurnace needs four meters. On the other hand, when the length L of theannealing furnace has only two meters, it is revealed that it isnecessary to set the drawing rate v to 5 m/second. However, from theviewpoint of productivity, for example, the drawing rate v is selectedin a range of about 10 m/second to 50 m/second, the length L of theannealing furnace is selected in a range of about one meter to tenmeters, and the slow cooling time t is one second or less.

<Rapid Cooling Process P4>

After the slow cooling process P3, the optical fiber is covered with acoating layer to enhance the resistance against external flaws, forexample. Typically, this coating layer is formed of an ultravioletcurable resin. In order to form such a coating layer, it is necessary tosufficiently cool the optical fiber at a low temperature for preventingthe coating layer from being burn, for example. The temperature of theoptical fiber affects the viscosity of a resin to be applied, and as aresult, this affects the thickness of the coating layer. A suitabletemperature of the optical fiber in forming the coating layer isappropriately determined suitable for the properties of a resin formingthe coating layer.

In the optical fiber production method according to one or moreembodiments, since the annealing furnace 121 is provided between thedrawing furnace 110 and a coater 131, the section for sufficientlycooling the optical fiber is decreased. More specifically, the opticalfiber production method according to one or more embodiments alsoincludes the precooling process P2, further decreasing the sectionsufficiently cooling the optical fiber. Thus, the optical fiberproduction method according to one or more embodiments includes therapid cooling process P4 in which the optical fiber delivered out of theannealing furnace 121 is rapidly cooled using a cooling device 122. Inthe rapid cooling process P4, the optical fiber is rapidly cooled fasterthan in the slow cooling process P3. Since the rapid cooling process P4performed in this manner is provided the temperature of the opticalfiber can be sufficiently decreased in a shorter section, easily formingthe coating layer. The temperature of the optical fiber when it isdelivered out of the cooling device 122 is in a range of temperatures of40° C. to 50° C., for example.

As described above, the optical fiber, which has been passed through thecooling device 122 and cooled to a predetermined temperature, is passedthrough a coater 131 containing an ultraviolet curable resin to be thecoating layer that covers the optical fiber, and the optical fiber iscovered with this ultraviolet curable resin. The optical fiber isfurther passed through an ultraviolet irradiator 132, ultraviolet raysare applied to the optical fiber, the coating layer is formed, and thenthe optical fiber 1 is formed. Note that, the coating layer is typicallyformed of two layers. In the case of forming a two-layer coating layer,after the optical fiber is covered with ultraviolet curable resinsforming the respective layers, the ultraviolet curable resins are curedat one time, and then the two-layer coating layer can be formed.Alternatively, after forming a first coating layer, a second coatinglayer may be formed. The direction of the optical fiber 1 is changed bya turn pulley 141, and then the optical fiber 1 is wound on a reel 142.

As described above, the present invention is described with reference toone or more embodiments taken as examples. The present invention is notlimited to the above described embodiments. That is, the optical fiberproduction method according to one or more embodiments of the presentinvention only has to include the drawing process and the slow coolingprocess described above. The precooling process and the rapid coolingprocess are not essential processes. The optical fiber production methodaccording to one or more embodiments of the present invention isapplicable to the manufacture of any types of optical fibers. Forexample, the optical fiber production method according to one or moreembodiments of the present invention is applicable also to productionmethods for optical fibers having different materials, such aschalcogenide glass and fluorine glass, as a principal component, as wellas production methods for optical fibers having silica glass as aprincipal component, if the constant A and the activation energy E_(act)in Equation (5) above are derived.

According to one or more embodiments of the present invention, there isprovided an optical fiber production method with which an optical fiberwith decreased transmission losses can be manufactured, and the methodcan be used in the field of optical fiber communications. The method canalso be used for fiber laser devices and other devices using opticalfibers.

REFERENCE SIGNS LIST

-   1 . . . optical fiber-   1P . . . optical fiber preform-   110 . . . drawing furnace-   111 . . . heating unit-   120 . . . tubular product-   121 . . . annealing furnace-   122 . . . cooling device-   131 . . . coater-   132 . . . ultraviolet irradiator-   141 . . . turn pulley-   142 . . . reel-   P1 . . . drawing process-   P2 . . . precooling process-   P3 . . . slow cooling process-   P4 . . . rapid cooling process

Although the disclosure has been described with respect to only alimited number of embodiments, those skilled in the art, having benefitof this disclosure, will appreciate that various other embodiments maybe devised without departing from the scope of the present invention.Accordingly, the scope of the invention should be limited only by theattached claims.

1. An optical fiber production method comprising: drawing an opticalfiber from an optical fiber preform in a drawing furnace; and coolingthe optical fiber, wherein the optical fiber is passed through aplurality of annealing furnaces while the optical fiber is cooled; andEquation (1) is held in a given period during the cooling, where a timeconstant of relaxation of a structure of glass forming a core includedin the optical fiber is defined as τ(T), a temperature of the opticalfiber at a point in time during the cooling is defined as T, a fictivetemperature of glass forming the core at the point in time is defined asT_(f) ⁰, and a fictive temperature of glass forming the core after alapse of time Δt from the point in time is defined as T_(f).20° C.<T _(f) −T=(T _(f) ⁰ −T)exp(−Δt/τ(T))<100° C.  (1)
 2. The opticalfiber production method according to claim 1, wherein Equation (2) isheld in a given period during the cooling.40° C.<T _(f) −T=(T _(f) ⁰ −T)exp(−Δt/τ(T))<60° C.  (2)
 3. The opticalfiber production method according to claim 1, wherein a relationship ofEquation (3) is held, where a preset temperature of an n^(th) annealingfurnace of the plurality of annealing furnaces from an upstream side isdefined as T_(sn) and a fictive temperature of glass forming a coreincluded in the optical fiber at an outlet port of the nth annealingfurnace of the plurality of annealing furnaces from the upstream side isT_(en).20° C.<T _(en) −T _(sn)<100° C.  (3)
 4. The optical fiber productionmethod according to claim 3, wherein Equation (4) is held.40° C.<T _(en) −T _(sn)<60° C.  (4)
 5. The optical fiber productionmethod according to claim 1, wherein a temperature difference between apreset temperature and a fictive temperature of glass forming the coreat an outlet port is smaller in the annealing furnace provided on adownstream side than in the annealing furnace provided on an upstreamside.
 6. The optical fiber production method according to claim 1,wherein the optical fiber is in any one of the plurality of annealingfurnaces during at least certain period for which a temperature of theoptical fiber is greater than or equal to 1,300° C. and less than orequal to 1,500° C.